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Generating Economic Cycles 



the electric current are sealed into the glass of the tube, 

 forming electrodes at the points A and C. The end of 

 the wire where the current enters the tube, A, is the 

 anode, and the other end, C, where the current makes 

 its exit is the cathode. The electrodes are usually 

 tipped with some substance, such as alimiinium, that 



Figure 27. A simple discharge tube. 



does not easily volatilize under an electric discharge. 

 The degree of vacuum that is obtained is measured 

 by the pressure of the gas within the tube. 



Under the conditions of ordinary pressure, air and 

 other gases are among the best insulators of electricity. 

 But when a highly rarefied gas is confined within a 

 discharge tube and the electric tension between the 

 electrodes is great, the gas becomes a conductor and 

 reveals the remarkable phenomena that have led to the 

 recent views as to the nature of electricity. If, for 

 example, a tube with anode and cathode about twelve 

 inches apart is filled with air at ordinary pressure, 

 no spark will pass between the electrodes even when 

 the tension is comparatively high. When, however, 

 the pressure is reduced to about one millimetre of 

 mercury, a glow appears at the two electrodes, the 

 positive glow being greater than the glow at the 



